Waveguide directional filter employing quarter-wave spaced parallel tuned cavities



CHU-QUON LEE 3,200,352 WAVE Aug. 10, 1965 WAVEGUIDE DIRECTIONAL FILTER EMPLOYING QUARTER- SPACED PARALLEL TUNED CAVITIES 2 Sheets-Sheet 1 Filed May 11, 1962 M. w m a O 3 3 A m0. A V 2 O 2 m L I B E 2 m 9 6 w w 3 ,I 3 m 5 W F A B B 6 A d e e e l w a A R R R F 9 N 2 2 Th K Q Md w .m 3 flalnwlslmm I2 INVEN TOR C/m 0110/1 Lee Aug. 10,1965 CHU'QUON LEE 3,200,352

WAVEGUIDE DIRECTIONAL FILTER EMPLOYING QUARTER-WAVE SPACED PARALLEL TUNED CAVITIES 2 Sheets-Sheet 2 Filed May 11, 1962 INVENTOR Ulla Ouon [66 FIG 5 United States Patent 3,200,352 WAVEGUIDE DREQTIUNAL FILTER EMPLOYHNG QUARTER-WAVE SPAtIED PARALLEL TUNED CAVJITIES Chu-Quon Lee, Maywood, 111., assignor to Motorola, inc, Chicago, 11]., a corporation of Illinois Filed May 11, 1962, Ser. No. 193,980 Claims. (Cl. 3333-) This invention relates in general to electrical coupling devices and more particularly to directional filters for waveguide transmission lines.

Directional filters are used with waveguide or coaxial structures operating at frequencies in the microwave region in many applications. Such devices are used to couple electromagnetic signal waves from one transmission line to another transmission line such that in one direction the induced waves are effectively cancelled while in the other direction they are reinforced. This characteristic of directivity is useful in microwave systems in permitting more than one transmitter and/or receiver to be supplied from a common transmission line and antenna system with minimum interaction of signals. Directional filters may also be usefully employed in microwave components, as in the receiver, to couple signals from the local oscillator stage to the mixer stage while preventing such signals from feeding back into the preselector stage. Because it has low insertion loss, a directional filter is an exceptionally good coupler for a low level local oscillator such as a solid state oscillator.

Many types of directional filters are known in the art. All such prior devices such as the type lmown as the circularly polarized directional filter have required four separate ports. These devices are bulky, complex in structure, and are not readily adaptable for inclusion as an integral part of a waveguide structure from an economic view point.

Accordingly, an object of the present invention is to provide a waveguide directional filter having improved directivity characteristics and reduced insertion loss.

A further object is to provide a waveguide directional filter which is simple in mechanical and electrical design and readily adaptable for inclusion as an integral part of a waveguide structure to provide an overall compact mechanical package.

Another object is to provide a simple directional filter unit for a waveguide transmission line wherein but three ports are required.

A feature of the present invention is the provision of a directional filter for mechanical afiixation to a length of rectangular waveguide and which includes an input port for applying a source of signal waves to first and second tuned cavities in common, each cavity being electrically coupled to the waveguide by associated coupling apertures located in the wall common to the filter cavities and waveguide. The centers of the apertures are spaced approximately one quarter wavelength apart, and the cavities are tuned to frequencies whereby the signal waves coupled into the waveguide are respectively 45 degrees leading and lagging the reference frequency of the signal wave source thereby effecting phase quadrature, so that the coupled waves are reinforced in the main waveguide by constructive addition in one direction while cancelling by destructive interference in the other direction.

Another feature of the invention is the provision of such a directional filter wherein the tuned cavities each include a pair of elongated slots having their long di mensions parallel to the longitudinal axis of the waveguide and therefore electrically coupling to the main waveguide, with the slots being dimensioned to properly impedance match the admittance of the tuned cavities to the characteristic admittance of the waveguide for improved operating characteristics and reduced insertion loss.

In the drawings:

FIG. 1 is a block diagram of a system employing the present invention whereby a plurality of receivers may be selectively coupled to a common waveguide transmission line;

FIG. 2 is a block diagram of a microwave receiver employing the present invention to couple the local oscillator of the receiver to the mixer stage;

i6. 3 is a perspective view of the directional filter unit;

FIG. 4 is a view of the unit of FIG. 3 partly in cross section with the top wall removed;

FIG. 5 is a top cross section view showing the position of the coupling slots; and

FIG. 6 is an equivalent circuit with the associated rotational vectors representing electromagnetic waves at different reference planes.

In practicing the invention a directional filter coupling device is provided whereby electromagnetic signal waves may be coupled into a waveguide transmission line in a particular direction. The directional filter consists of an input port feeding a pair of tuned cavities in common, each of which is coupled to the waveguide transmission line by a pair of slot apertures. The cavities are tuned to frequencies whereby the respective waves coupled into the waveguide are degrees lagging and 45 degrees leading the reference frequency of the signal wave source, thereby being in phase quadrature with each other. The centers of the slot apertures in one tuned cavity are spaced approximately a quarter wavelength from the centers of the slot apertures in the other tuned cavity so that the waves coupled into the main waveguide are reinforced in one direction by vector addition while cancelling in the other direction by destructive interference. Further, the inductive irises and the slot apertures in each tuned cavity are positioned in a manner, and are of a proportionate size, to properly impedance match the input port to the main waveguide to improve operating characteristics and reduce insertion loss.

Referring to the drawings, FIG. 1 illustrates a typical use for the direction filter of the invention. A plurality of receivers, designated by numerals 6, 7 and 8, may be coupled to waveguide manifold 2 by associated directional filters 3, 4 and 5. Each of the receivers may be tuned to a particular frequency f f and f respective- 1y. These different signals are all received by antenna 1 and are then directionally coupled by the directional filters 3, 4 and 5 to the respective receivers. This minimizes interaction of signals which would otherwise produce spurs and reflections, as well as eliminates the necessity of phase-shifters which are required in standard multiplex systems. The manifold 2 is terminated in a matched load 9.

FIG. 2 shows in block diagram form the incorporation of the present invention in a typical microwave receiver such as the receiver 6 of FIG. 1. Received electromagnetic waves in the waveguide manifold 2 are di-' rected into the preselector stage it) of receiver 6 through directional filter 3. The output of receiver preselector stage it) is coupled to the receiver mixer stage 12 through waveguide section 13. The receiver local oscillator stage 14 is further coupled to mixer stage 12 through the directional filter 15. Directional filter 15 is constructed in accordance with the present invention to conple signal energy into waveguide 13 in one direction only, i.e., to mixer stage 12. This characteristic of directivity provides a desirable high degree of isolation between local oscillator 14 and the preselector stage 10.

FIG. 3 is a perspective view of the directional filter unit. Waveguides 16 and 17 are hollow-pipe guide me bers of conductive material, with a rectangular cross section as is commonly used in the transmission of electromagnetic waves. Tuning screws 13 are provided for tuning cavities within the filter as will be explained.

It can be seen from FIG. 4 that the directional filter unit includes a pair of tuned cavity chambers 19 and 20 separated by a common wall 21. A partial transverse partition 22 is positioned in the path of the signal waves from waveguide 16 to form a pair of inductive irises 23 and 24 coupling signal waves to cavities 19 and 2%, respectively. Because of the symmetry of the irises, and of the coupling slots to be described, electromagnetic signal waves feeding into waveguide section 16 will divide equal- 1y into cavities l9 and 2%. Thus the signal waves applie to tuned cavities 19 and 26 are in phase.

Cavities 19 and 20 are mechanically afiixed to waveguide section 17 in a manner such that the longitudinal dimension of the cavities is transverse to the longitudinal dimension of the Wider wall of waveguide section 17. Each of the cavities 19 and 26 is electrically coupled to waveguide section 17 by a pair of identical slot apertures located in the wall common to the cavities and waveguide.

As best shown in FIG 5, the coupling apertures are substantially elongated slots having their long dimensions parallel to the longitudinal axis of waveguide 17. All of the coupling apertures -23 are identical in physical dimension, and are positioned such that the centers of apertures 25 and 27 in cavity 19 are spaced approximately one-quarter wavelength from the centers of apertures 26 and 28 in cavity 26. These apertures couple the respective cavities 19 and 29 in shunt with waveguide section 17 by virtue of the apertures cutting the transverse current of the propagated electromagnetic waves.

FIG. 6 is the equivalent circuit of the directional filter of FIGS. 3, 4 and 5. The input waves to tuned cavities 19 and 20 are in phase with each other and are each represented in magnitude and phase by the rotating vector 29. Because the cavities 19 and 20 are symmetrically coupled into waveguide 16 through identical irises, power input into the waveguide 16 will be equally divided into the cavities 19 and 20 when they are properly tuned. The rotating vectors 30 and 31 in the reference planes A and B represent waves coupled into Waveguide 17 from the cavities 19 and 7.0, respectively. The output waves 30 and 31 are of equal magnitude but they are degrees leading and lagging respectively with reference to the input voltage in waveguide 16. To achieve a phase quadrature between these output waves, cavities 19 and 20 are tuned by screws 18 (FIG. 3) to respective frequencies above and below the frequency of the input wave.

Under this condition, the admittance looking into the associated irises of cavities 1? and 20 will be conjugates of each other. Since cavities 19 and 20 are shunt connected to waveguide section 16, the total admittance presented to waveguide 16 will be the sum of the admittances due to cavities 19 and 20. Therefore the total admittances of cavities 19 and 20 will be real and can be optimumly matched to the characteristic admittance of waveguide 16 by selecting proper dimensions for irises 23 and 24, and the slot apertures 25-28. Proper impedance matching thereby improves the overall operating characteristics and reduces the inherent insertion loss.

The waves applied to the waveguide 17 from the cavities 19 and 20 Will divide and pass in both directions in the waveguide 17. Considering the wave applied to the waveguide 17 from cavity 19 and represented by vector 30, this will divide with the part of the Wave represented by vector 30a passing to the right and an equal part of the wave represented by vector 3% passing to the left as shown in FIG. 6. Similarly, the wave 31, from cavity 20 will divide with a portion 31a of the wave passing to the right and a similar portion 31b passing to the left. Considering the wave going to the right at reference plane B, the portion 30a from cavity 19 will be rotated through degrees as it moves from reference plane A to reference plane B so that at reference plane B the waves 30a and 31a will have opposite phases and will cancel. Therefore, there will be no Wave introduced into the guide 17 by the cavities 19 and 20 which will pass to the right. Considering the wave going to the left at reference plane A, the portion 31b from cavity 20 will be rotated through 90 degrees as it moves from reference plane B to reference plane A, and will be in phase with the portion 30b injected from cavity 19 and moving to the left. Accordingly, these two waves Will reinforce or add as shown by the vectors 30b and 31b at reference plane A to provide a strong wave moving to the left.

It is emphasized that the present invention provides a directional filter having but three ports as compared with the .common requirement of four ports in prior devices. In the present invention a first input port is provided by waveguide 16, and second and third output ports are provided in the two ends of waveguide section 17. This provides a simple compact mechanical assembly suitable for adaptation as an integral part of the overall Waveguide structures of a receiver or other unit.

Thus the present invention provides a directional filter device with improved operating characteristics and of simplified design. Mechanical space requirements are significantly reduced as well as cost consideration in the fabrication and manufacture. A high degree of directivity is realized with reduced insertion loss.

I claim:

1. A device for directionally coupling a signal wave of a predetermined frequency into a conducting waveguide of rectangular cross section including in combination, filter means forming first and second tuned cavities having a common wall therebetween, means connecting said filter means to a large dimension side of the waveguide, said Waveguide having apertures therein coupling each of said cavities to the waveguide, and input means coupled to said first and second tuned cavities for applying the signal wave thereto, said cavities being coupled in parallel whereby the signal wave is applied to each of said cavities at the same input phase, each of said cavities being tuned to a frequency related to the predetermined frequency of the signal Wave whereby the respective signal wave portions coupled into the Waveguide from said cavities have different phases with respect to each other, said cavities being tuned and said aperture being positioned so that said signal wave portions coupled into the waveguide from said first and second cavities cancel in one direction and reinforce in the other direction.

2. Directional filter apparatus for operating at a predetermined frequency and including in combination, a length of rectangular waveguide transmission line, waveguide means forming first and second tuned cavities having a first common wall therebetween, means for connecting said waveguide means to said waveguide transmission line along a side of said line having the larger dimension, said transmission line and said waveguide means having a second comm-on wall with a pair of spaced slot apertures located in said second common wall for coupling each cavity to said transmission line, one of said apertures being symmetrically positioned in each of said cavities and the center of said aperture in said first cavity being displaced from the center of said aperture in said second cavity by approximately one-quarter wavelength at said predetermined frequency, and means for applying signal Waves to said first and second cavities in common, each of said cavities being tuned to a frequency related to said predetermined frequency so that the signal waves coupled into said waveguide transmission line by said cavities are 45 degrees leading and 45 degrees lagging respectively with reference to the applied signal Waves, said signal waves coupled into said waveguide transmis- 5 sion line being in phase quadrature with respect to each other to effectively cancel by destructive interference in one direction While reinforcing by constructive addition in the other.

3:. Directional filter apparatus for operating at a predete'mined frequency and including in combination, a first length of rectangular Waveguide line having first and second ports, waveguide means forming first and second tuned cavities having a first common Wall therebetween and having a third port for applying signal waves to said cavities in common, means for connecting said Waveguide means to said waveguide line along a side of said line havin the larger dimension, said Waveguide line and said Waveguide means having a second common Wall therebetween having a pair of slot apertures therein to electrically couple each of said cavities to said Waveguide line, said apertures having their centers displaced approx imately one-quarter Wa elength apart along the longitudinal axis of said waveguide line and means for applying signal waves to said third port, each of said cavities being tuned to a frequency related to said predetermined frequency so that the signal Waves coupled into said Waveguide line by said cavities are in phase quadrature with reference to each other at said apertures, said phase quadrature relation of said Waves and said quarter Wave spacing of said apertures causing said signals to be added at one of said first and second ports and to be cancelled at the other of said first and second ports.

4. Directional filter apparatus including in combination, a length of rectangular Waveguide transmission line having a given characteristic admittance, waveguide means forming first and second tuned cavities having a first common Wall therebe'tween and each presenting a given admittance, means for connecting said cavity forming means to a larger dimension side of said Waveguide transmission line, said waveguide line and said Waveguide means having a second comm-on wall including a pair of elongated apertures between each of said cavities and said Waveguide line with their long dimensions being parallel to the longitudinal axis of said Waveguide line, said apertures being positioned to provide symmetrical shunt coupling to said waveguide line and having dimensions to impedance match the total admittance of said cavities to the characteristic admittance of said Waveguide line, and means for applying signal waves at a given frequency to said waveguide means forming said cavities, each of said cavities being tuned to a frequency related to said given frequency so that the signal waves coupled into said waveguide transmission line from said cavities are 45 degrees leading and 45 degrees lagging respectively with reference to said applied signal Waves, said signal Waves coupled into said waveguide transmission line being in phase quadrature to effectively cancel by destructive interference in one direction and reinfiorcing each other by constructive addition in the other direction.

5. Directional filter apparatus for operating at a predetermined frequency and including in combination, a length of rectangular waveguide transmission line, waveguide means forming first and second tuned cavities having a first common wall therebe tween, means for conneoting said waveguide means to said Waveguide transmission line along a side of said line having the larger dimension, said transmission line and said waveguide means having a second common Wall with a pair of spaced apertures located in said second common wall for coupling each cavity to said transmission line, said apertures being symmetrically positioned in said cavities with the center of said aperture in said first cavity being displaced from the center of said aperture in said second cavity by approximately one quarter wave length at said predetermined frequency, and means for applying signal Waves to said first and second cavities in common, one of said cavities being tuned to a frequency above the predetermined frequency and the other of said cavities being tuned .to a frequency below said predetermined frequency so that the signal waves coupled into said Waveguide line by said cavities are in phase quadrature by being 45 degrees leading and 45 degrees lagging respectively with reference to the applied signal Waves, said signal Waves coupled in phase quadrature into said Waveguide transmission line and spaced one quarter Wavelength by said apertures effectively cancelling by destructive interference in one direction by being directly out of phase at one aperture while reinforcing by constructive addition in the other direction by being in phase at said other aperture.

References Cited by the Examiner UNITED STATES PATENTS 2,434,646 1/48 Fox 33373 2,961,619 11/60 Breese et al. 33-3-10 HERMAN KARL SAALBACH, Primary Examiner. 

1. A DEVICE FOR DIRECTIONALLY COUPLING A SIGNAL WAVE OF A PREDETERMINED FREQUENCY INTO A CONDUCTING WAVEGUIDE OF RECTANGULAR CROSS SECTION INCLUDING IN COMBINATION, FILTER MEANS FORMING FIRST AND SECOND TUNED CAVITIES HAVING A COMMON WALL THEREBETWEEN, MEANS CONNECTING SAID FILTER MEANS TO A LARGE DIMENSION SIDE OF THE WAVEGUIDE SAID WAVEGUIDE HAVING APERTURES THEREIN COUPLING EACH OF SAID CAVITIES TO THE WAVEGUIDE, AND INPUT MEANS COUPLED TO SAID FIRST AND SECOND TUNED CAVITIES FOR APPLYING THE SIGNAL WASVE THERETO, SAID CAVITIES BEING COUPLED IN PARALLEL WHEREBY THE SIGNAL WAVE IS APPLIED TO EACH OF SAID CAVITIES AT THE SAME INPUT PHASE, EACH OF SAID CAVITIES BEING TUNED TO A FREQUENCY RELATED TO THE PREDETERMINED FREQUENCY OF THE SIGNAL WAVE WHEREBY THE RESPECTIVE SIGNAL WAVE PORTIONS COUPLED INTO THE WAVEGUIDE FROM SAID CAVITIES HAVE DIFFERENT PHASES WITH RESPECT TO EACH OTHER, SAID CAVITIES BEING TUNED AND SAID APERTURE BEING POSITIONED SO THAT SAID SIGNAL WAVE PORTIONS COUPLED INTO THE WAVEGUIDE FROM SAID FIRST AND SECOND CAVITIES CANCEL IN ONE DIRECTION AND REINFORCE IN THE OTHER DIRECTION. 